1. Field of the Invention
The present invention relates to communication systems for underground mines and in particular, radio communication systems operating within the medium frequency range to provide radio coverage to mobile personnel within the mine via parasitic coupling into and from existing mine conductors.
2. Description of Prior Art
Although radio transmission on the surface of the earth is well understood, radio transmission in an underground environment has posed many problems. Complex interactions occur between the radio waves and the underground environment. Characteristics of the geology (stratified layering, boundering effects, conductivity, etc.) and the mine complex (entry dimensions, conductors, electromagnetic interferences, etc.) must be measured and understood even before a practical mine radio communication system can be built. To this end, considerable research has been conducted. For example, Arthur D. Little, "Propagation of Radio Waves in Coal Mines, Task Order 1, Task Order F," BuMines Open File Report 46-77, 1977.
In a confined area such as a mine, a radio wave can propagate a useful distance only if the environment has the necessary electrical and physical properties. The "environment" takes into account the natural geology and man-made perturbations such as the mine complex itself. For example, if the wave length of a radio wave is small compared to the entry dimensions, a wave guide mode of propagation is possible. Attenuation depends primarily upon the physical properties of the entry such as cross-sectional area, wall roughness, entry tilts, and obstacles in the propagation path. Secondary effects such as the dielectric constants in earth conductivity also influence attenuation. Mine radio systems based upon this effect are available commercially. These are UHF systems operating around 450 MHz which provide useful but limited coverage. In high coal line-of-site, ranges of 300 meters are often possible. In nonline-of-site, such as going around a coal pillar, range is reduced severely. In lower coal, or when obstacles exist in the propagation path, range is reduced even more. For this reson, a UHF radio system requires an extensive network of leaky feeder transmission cables and repeaters to become useful. Even so, range from the cable is not usually great, and equipment cost is very high. Medium frequency (MF) signals, i.e., in the range of 300 kHz-3 MHz, and standard mine carrier frequency signals, i.e., 61 kHz-190 kHz, propagate in the underground environment.
The Chamber of Mines of South Africa made various attempts to improve underground mine radio communications. They have made efforts to develop radio systems for deep mines, primarily gold mines. See Austin, B. A., "Underground Radio Communication Techniques and Systems in South African Mines"; Proceedings, Electromagnetic Guided Waves in Mine Environments Workshop, Golden, Colo., 1978, pp. 87-102. Consequently, a 1-W SSB portable radio system was developed. However, performance of these units in U.S. coal mines was not satisfactory for various reasons. First, U.S. mines are highly mechanized with electrical equipment, thereby producing considerable electromagnetic interference (EMI) which desensitizes single sideband (SSB) radios. Secondly, the units which were produced were not of sufficient power. U.S. mines are mostly room and pillar, which means that any radio system must have reasonable range from local conductors. Furthermore, the electrical parameters of the geology is less favorable in the United States than in gold mines of South Africa.
Many tests have been conducted to determine the actual propagation characteristics of medium frequency (MF) signals underground and to relate the propagation to the underground environment, such as geology, entry size, existing conductors, EMI, etc. For example, see Lagace, R. L., A. G. Emslie, and M. A. Grossman; "Modeling and Data Analysis of 50-5000 kHz Radio Wave Propagation in Coal Mines," Task Order 4, BuMines Contract HO346045, Arthur D. Little, Inc., BuMines Open File Report 83-80, 1980.; Corey, T. S., "Propagation of EM Signal in Underground Mines"; BuMines Contract HO366028, Collins Radio Group; BuMines Open File Report 136-78, 1977; Collins Commercial Telecommunications, "Electromagnetic Noise and Propagation in Low Coal Mines at Medium Frequencies;" Contract HO377053; BuMines Open File No. 63-82, 1982. These programs formed a foundation for an understanding of how medium frequency signals propagate in a stratified medium of various electrical parameters and which are often interlaced by man-made conducting structures (rails and power lines) in artificial voids (entry ways).
In the mining of minerals, the productivity, efficiency and safety depend a great deal on communications between the miners underground with each other and with surface stations. Also, it is necessary, from an economical viewpoint, to provide a radio communication system which does not require special or dedicated communications cable within the mine. Also, it is necessary to have radio communications between miners while they are working and mobile. As a result thereof, it is necessary to have communication equipment on the miners' bodies which does not interfere with the miners' mining activities. Communication is also desirable between underground mobile mining equipment, the individual miners and the ground stations. Furthermore, it is necessary for such units to operate at low electrical power since safety requires that equipment not be prone to creating sparks which may cause destructive events.